Compact and efficient spectrometers are essential for biological and environmental sensing applications in which optical signals of interest are usually very weak and portability is highly desired.
A key element of a spectrometer is a wavelength sensitive (or dispersive) device that allows for separation of different wavelength channels for detection. Holograms (or gratings) are well-known candidates for this task due to their wavelength selectivity, which results in non-uniform diffraction of different wavelength channels of a collimated optical beam. Most of the optical spectrometers built based on this phenomenon exploit surface relief or thin film gratings which primarily have single grating vectors. However, these spectroscopy techniques are not efficient for spatially incoherent light sources.
The reason is that for an incoherent source with uniform spectrum in the input plane of such spectrometers, the output will be an ambiguous pattern with contributions from different wavelength channels overlapping each other. The problem has been solved in conventional spectrometers by limiting the angular range of the incident beam by using spatial filtering. Unfortunately, spatial filtering drastically reduces the photon throughput for diffuse source spectroscopy. While such inefficiency might be tolerated in absorption spectroscopy (where a strong incoherent source can be used), it is a major limitation for weak diffuse sources, such as those generated in Raman spectroscopy. In such cases, the signal from the desired molecules is very weak and successful sensing requires a sensitive and efficient spectrometer.